1. Field of the Invention
The present invention relates to an antistatic pneumatic tire showing improved durability.
2. Description of the Related Art
Heretofore, carbon black has generally been used as the reinforcing agent for tires. However, in recent years, improvement in the fuel economy of automobiles has been demanded to deal with environmental problems and much study has centered on how to decrease the rolling resistance of a tire. Therefore, tread rubbers having formulations containing a decreased amount of carbon black which causes hysteresis loss and using silica as a reinforcing material have been developed. Tread rubbers comprising silica as a reinforcing material provide increased braking ability on wet roads (referred to as wet traction, hereinafter) and decreased rolling resistance. However, tread rubbers comprising silica have a problem in that the electric resistance thereof is higher than that of tread rubbers comprising carbon black without silica, and electricity conducted from the body of an automobile and electricity generated by internal friction during deformation of the rubber are accumulated.
To solve the problem of static electricity accumulation, for example, antistatic tires having a surface covered with a layer of a conductive rubber have been proposed in the specifications of European Patent No. 705722 and European Patent No. 718126. However, these tires have a problem in that the electric conductivity decreases when abrasion of the tread rubber takes place in the intermediate to final period of the life span of the tire.
In the specification of European Patent No. 681931, an antistatic pneumatic tire comprising a conductive rubber member disposed in the circumferential direction of the tire has been proposed. In the specifications of the U.S. Pat. No. 5,518,055 and European Patent No. 658,452, antistatic tires comprising two conductive rubber member disposed between the tread portion and the sides of the tires have been proposed. However, these tires have a problem in that the tires are weak against a force applied from a transverse direction because these tires have a structure shown in FIG. 6 in which conductive rubber members 20 having the same thickness as that of tread rubbers 10 are disposed between the tread portions 10. Separation tends to takes place at the boundaries of the conductive rubber members 20 and the tread portions 10 as shown in FIG. 7. When separation takes place at a portion of the tire, the separation extends in the circumferential direction over the whole tire, and the durability of the tire markedly decreases.
The present invention has been made in accordance with the above facts and has an object of providing a pneumatic tire showing not only excellent static discharge effects but also excellent durability.
The present inventors paid attention to the arrangement of conductive rubber members, and it was found as the result of extensive studies that the above object can be achieved by the means explained below. The present invention has been made on the basis of this knowledge.
The pneumatic tire of the present invention is described.
The pneumatic tire of the present invention comprises a tread portion which comprises a surface rubber layer which is disposed in an outer portion of the tread portion in the radial direction of said tire and has a specific resistance of 108 xcexa9xc2x7cm or more after curing by vulcanization, and conductive rubber members which comprise a rubber composition having a specific resistance of 106 xcexa9xc2x7cm or less and with which portions of the surface rubber layer are replaced, wherein the conductive rubber members are disposed in portions of the surface rubber layer extending from the outer surface to the inner surface in the radial direction of said tire discontinuously in the circumferential direction of said tire at the outer surface of the surface rubber layer in the radial direction of said tire.
The pneumatic tire of the first embodiment of the present invention comprises a tread portion which comprises a surface rubber layer which is disposed in an outer portion of the tread portion in the radial direction of said tire and has a specific resistance of 108 xcexa9xc2x7cm or more after curing by vulcanization, and conductive rubber members which comprise a rubber composition having a specific resistance of 106 xcexa9xc2x7cm or less and with which portions of the surface rubber layer are replaced, wherein the conductive rubber members are disposed in portions of the surface rubber layer extending from the outer surface to the inner surface in the radial direction of said tire, are discontinuous in the circumferential direction of said tire at the outer surface of the surface rubber layer in the radial direction of said tire, have a width of 0.1 to 3.0 mm, and are disposed in such a manner that at least some of the conductive rubber members are placed inside an area of the surface of said tire which contacts with a road surface.
In this pneumatic tire, it is preferable that the conductive rubber members are disposed at at least three positions which are placed in an area of the surface of said tire in contact with a road surface and are placed in the same cross section in the transverse direction of said tire.
It is preferable that the length of the conductive rubber member in the circumferential direction of said tire is the same as or less than the length of an area of the surface of said tire in contact with a road surface. It is further preferable that the length of the conductive rubber member in the circumferential direction of said tire is {fraction (1/12)} or less of the length of the circumference of said tire. It is also preferable that the conductive rubber members have a width of 0.5 to 2.0 mm.
The tread portion may have a structure consisting of at least two layers comprising a surface rubber layer which is disposed in an outer portion of the tread portion in the radial direction of said tire and an inner rubber layer which is disposed at an inner portion of the tread portion in the radial direction of said tire and has a specific resistance of 106 xcexa9xc2x7cm or less after curing by vulcanization.
It is preferable that the conductive rubber members are made from a rubber cement having a specific resistance of 106 xcexa9xc2x7cm or less after curing by vulcanization.
The pneumatic tire of the first embodiment of the present invention is characterized in that, when the conductive paths are formed by disposing conductive rubber members in the tread portion, the conductive rubber members are separated from each other in the circumferential direction with portions of the tread disposed between the conductive rubber members in the circumferential direction, and not in such a manner that a continuous single conductive rubber member is formed around the complete circumference.
Because of the above structure, force in the transverse direction of the tire is effectively dispersed, and movement of the conductive rubber members and deformation of the rubber at the boundaries thereof is suppressed preventing separation of the conductive rubber members. As a result, durability of the tire is improved.
The pneumatic tire of the second embodiment of the present invention comprises a tread portion which comprises a surface rubber layer which is disposed in an outer portion of the tread portion in the radial direction of said tire and has a specific resistance of 108 xcexa9xc2x7cm or more after curing by vulcanization and conductive rubber members which comprise a rubber composition having a specific resistance of 106 xcexa9xc2x7cm or less, have the shape of a column having a small diameter, and replace portions of the surface rubber layer, wherein the conductive rubber member having a shape of a column having a small diameter are disposed in portions of the surface rubber layer extending from the outer surface to the inner surface in the radial direction of said tire and are scattered discontinuously on the outer surface of the surface rubber layer in the radial direction of said tire, preferably at locations separated by approximately the same distance.
It is preferable in the pneumatic tire that thirty or more of the conductive rubber members having the shape of a column having a small diameter are disposed in an area of the surface of said tire which contacts the road surface. It is also preferable that the shape of a column having a small diameter is a shape of an approximately round column and has a diameter of 0.1 to 3 mm. It is more preferable that the shape of an approximately round column has a diameter of 0.5 to 2.0 mm.
The tread portion may have a structure consisting of at least two layers comprising a surface rubber layer which is disposed in an outer portion of the tread portion in the radial direction of said tire and an inner rubber layer which is disposed at an inner portion of the tread portion in the radial direction of said tire and has a specific resistance of 106 xcexa9xc2x7cm or less after curing by vulcanization.
It is preferable that the conductive rubber members are made from a rubber cement having a specific resistance of 106 xcexa9xc2x7cm or less after vulcanization.
The pneumatic tire of the second embodiment of the present invention is characterized in that, when the conductive paths are formed by disposing conductive rubber members in the tread portion, conductive rubber members having the shape of a column are disposed across the entire circumference of the tire at locations separated from each other by approximately the same distance so that many locations of the conductive rubber members are disposed in the area of the surface of said tire which contacts the road surface.
In the same manner as that of the pneumatic tire of the first embodiment of the present invention, force in the transverse direction of the tire is effectively dispersed, and movement of the conductive rubber members and deformation of the rubber at the boundaries thereof is suppressed preventing separation of the conductive rubber members because of the above structure. As a result, the durability of the tire is improved.
The pneumatic tire of the third embodiment of the present invention comprises a tread portion which comprises a surface rubber layer which is disposed in an outer portion of the tread portion in the radial direction of said tire and has a specific resistance of 108 xcexa9xc2x7cm or more after curing by vulcanization and conductive rubber members which comprise a rubber composition having a specific resistance of 106 xcexa9xc2x7cm or less and replace portions of the surface rubber layer, wherein the conductive rubber members are disposed in portions of the surface rubber layer extending from the outer surface to the inner surface in the radial direction of said tire and are discontinuous in the circumferential direction of said tire at the outer surface of the surface rubber layer in the radial direction of said tire and continuous in the circumferential direction of said tire at an inner portion of the tread portion in the radial direction of said tire.
In the present pneumatic tire, it is preferable that the surface area of the conductive rubber members exposed at the outer surface of the tread portion is 1 to 50% of the surface area of the conductive rubber members if the conductive rubber members were extended continuously in the circumferential direction of said tire at the outer surface of the tread portion in the radial direction of said tire.
The pneumatic tire of the third embodiment of the present invention is characterized in that, as described above, the conductive rubber members comprising a rubber composition having a specific resistance of 106 xcexa9xc2x7cm or less are discontinuous in the circumferential direction of said tire at the outer surface of the tread portion in the radial direction of said tire and are continuous in the circumferential direction of said tire at an inner portion of the tread portion in the radial direction of said tire.
Because the conductive rubber members comprising a rubber composition having a specific resistance of 106 xcexa9xc2x7cm or less are discontinuous in the circumferential direction of the pneumatic tire at the outer surface of the tread portion in the radial direction of the tire, static electricity accumulation can be prevented, and even if separation occurs at a portion of the boundary between the conductive rubber members and the tread, spread of the separation along the circumference of the tire can be avoided thereby preventing a decrease in the durability of the tire. Moreover, because the conductive rubber members are continuously disposed in the circumferential direction of the tire at an inner portion of the tread in the radial direction of the tire, discharge of the electricity accumulated in the inner part of the tire can be facilitated. Because of these characteristics, the pneumatic tire of the present invention which shows not only the excellent effect of discharging electricity but also excellent durability has been obtained.
The pneumatic tire of the present invention shows an excellent ability to discharge electricity and does not allow static electricity to accumulate during the entire period of the life span of tire use because of the structures described above. Therefore, the generation of sparks can be prevented, and noise in automobile radios can be decreased. Moreover, separation of the tread portion and the conductive rubber members can be prevented by the effective dispersion of the force applied to the tire in the transverse direction, and the durability of the tire is remarkably improved.
The present invention includes a method of manufacturing a vulcanized tire having a tread portion and a layer radially beneath the tread portion wherein the tread includes a surface rubber layer, wherein the improvement comprises: forming cavities in the tread portion such that the cavities are provided discontinuously along the tire circumferential direction, wherein each cavity extends from a tire radial direction outer side surface of the tread portion to a tire radial direction inner surface of the tread portion; pouring and filling, into the cavities, a rubber cement; and vulcanizing the tread portion such that the rubber cement forms a plurality of conductive rubber members which are electrically connected to the layer beneath the tread portion, wherein the vulcanized surface rubber layer has a specific resistance of 108xcexa9xc2x7cm or more and the vulcanized conductive rubber members have a specific resistance of 106xcexa9xc2x7cm or less.